1. Field of the Invention
The present invention relates to an optical inspection apparatus and an optical inspection method. In particular, the present invention relates to an optical inspection apparatus and an optical inspection method that are applied, for instance, to a photomask (reticle) defect inspection apparatus used at the time of transfer of a circuit pattern of a semiconductor integrated circuit or the like.
2. Description of the Related Art
Lithography techniques capable of forming finer patterns are becoming necessary following an increase of the degree of integration of semiconductor integrated circuits. Against this backdrop, various methods are proposed and realized for detecting defects of pattern shapes of photomasks used for transferring semiconductor integrated circuit patterns on a semiconductor. As the photomasks u, there are binary masks and halftone masks. The binary masks are each a mask obtained by forming a light-shielding pattern made of a metallic film or the like on a glass substrate. The halftone masks are each a mask obtained by forming a pattern made of a material, which is semitransparent with respect to an exposure wavelength, on a glass substrate.
As the lithography techniques that enable formation of finer patterns, methods using phase-shift masks are proposed. The phase-shift masks are each a photomask obtained by adding (or burying) a material (phase shifter), which shifts the phase of exposure light, onto (or into) a glass substrate. With the phase-shift masks, high-resolution exposure is made possible through interference between light passing through a portion, to which the phase shifter is added, and light passing through a portion to which the phase shifter is not added. Known examples of the phase-shift masks include halftone masks (Att-PSMs: Attenuated Phase Shift Masks) and Levenson masks (Alt-PSMs: Alternating Phase Shift Masks).
In order to expose a fine pattern using such a phase-shift mask, it is important that the phase shifter on the phase-shift mask is consistent with design data. Therefore, in recent years, defect detection techniques for photomasks, such as the phase-shift masks, are desired earnestly and various apparatuses are proposed.
For instance, as a defect detection technique for photomasks utilizing light interference, a phase-shift amount measurement method using an optical heterodyne interference method is disclosed in JP 06-331321A (1994). Also, a phase-shift mask defect inspection method using a differential interference microscope is disclosed in JP 2002-287327 A.
Further, as a method utilizing diffraction/scattering of light, a photomask inspection method using analysis of a Fourier transformation image of a uniformly illuminated phase-shift pattern is disclosed in JP 04-229864 A (1992).
Still further, a method of detecting a defect of a surface of a wafer of a reticle or integrated circuit by causing only scattered/diffracted light from a phase-shift pattern illuminated from an inclined direction to pass through a spatial filter by means of a Fourier transformation surface is disclosed in JP 2002-519667 A.
On the other hand, as a method of detecting a pattern shape defect of a binary mask for which a light-shielding pattern is formed, a halftone mask for which a semitransparent pattern is formed, or the like, there is a scanning-type microscope system. This system is a system in which a pattern formation surface of a photomask is scanned with condensed light, and the intensity of transmitted light/reflected light is detected.
However, the conventional defect detection methods for photomasks, such as phase-shift masks, have the following problems (1) to (3).    (1) With the method using an optical heterodyne interference method and the method using a differential interference microscope described above, two light beams, whose positions are slightly displaced from each other, are irradiated onto a measurement target. Then, the intensity of interference between the two light beams are measured. Therefore, there is pattern direction dependence such as difficulty of detection of patterns extending in a direction that is the same as the direction of the displacement between the two beams. Also, there are problems such as limitation of detection of pattern line widths that depend on the amount of the displacement between the two beams.    (2) Also, the method described above that uses analysis of a Fourier transformation image of a uniformly illuminated phase-shift pattern is a method with which only a phase-shift amount (phase difference, film thickness) is basically measured. Therefore, the method is not aimed at detecting minute phase defects. Also, the method is devised based on analysis of a Fourier transformation image in a general imaging system using uniform illumination like in the case of a projection exposure apparatus. Therefore, there is a problem in that it is required to uniformly illuminate two areas that are an area, in which a phase shifter is added, and an area in which no phase shifter is added.    (3) Also, the method disclosed in JP 2002-519667 A described above, with which only scattered/diffracted light from a phase-shift pattern is detected with a Fourier transformation surface, is one method that is generally used in detection of defects of semiconductor wafers and the like. That is, the method is implemented by an apparatus that detects defects by measuring and analyzing scattered/diffracted light from a region dark-field/bright-field-illuminated by a light source such as a lamp or a laser. Various forms are devised, and inmost cases, it is aimed to improve the S/N of weak scattered/diffracted light from minute defects. For the S/N improvement and defect type judgment, JP 2002-519667 A also discloses a method with which detection is performed by performing spatial filtering on diffracted light in a far-field region. However, various scattered/diffracted light occurs depending on the pattern shape and defect shape of a detection target. In order to catch as much the light as possible, some consideration is needed for the angle of illumination light, the arrangement of a light reception system (or the kind of a spatial filter), and the like. Therefore, there is a problem in that it is required to construct a complicated system with which it is possible to cope with respective kinds of defects.
The problems described above are not limited to photomasks (reticles) and also apply to inspection of electronic component substrates, for which patterns are formed, in a like manner.